Electrolytic process for the separation of ions of amphoteric and non-amphoteric metals



sauna- Nov. 8, 1955 G. w. BODAMER 2,723,229 ELECTROLYTIC PROCESS FOR THE SEPARATION OF IONS 0F AMPHOTERIC AND NON-AMPHOTERIC METALS Filed Dec. 12, 1952 Anionic Cationic Permselecflve Permselective Membrane Membrane INVEN TOR. GEORGE W. BODAMER ATTORNEY United States Patent George W. Bodamer, Cheltenham, Pa., assignor to Rohm & Haas Company, Philadelphia, Pa., a corporation of Delaware Application December 12, 1952, Serial No. 325,686

4 Claims. (Cl. 204-98) This invention relates to an electrolytic process for the separation of ions of certain amphoteric and nonamphoteric metals. It relates to a method of electrolytically separating salts of certain non-amphoteric metals from salts of some amphoteric metals.

An object of this invention is to remove compounds of a non-amphoteric metal from mixtures containing compounds, usually salts, of both amphoteric and nonamphoteric metals. Another object is to provide a process for removing contaminating amphoteric compounds from aqueous solutions of salts of non-ampheteric metals or vice versa. A specific object is to remove ions of sodium from an aqueous solution containing both sodium and zinc ions.

These and other objects which will be apparent from the following disclosure are accomplished by the electrolytic process of this invention which comprises passing a direct electric current through an electrolysis cell which has an anode compartment containing hydrogen ions, a cathode compartment containing hydroxyl ions, and a third and intermediate compartment containing ions of both an amphoteric metal and a non-amphoteric metal, the partition between the anode compartment and the intermediate compartment being an anionic permselective diaphragm and the partition between the cathode compartment and the intermediate compartment being a cationic permselective diaphragm. Under these conditions the ions of the non-amphoteric metal migrate from the intermediate compartment into the cathode compartment while the ions of the amphoteric metal remain in the intermediate compartment.

The special kind of apparatus which is employed in the process of this invention can best be understood by a reference to the attached drawing wherein the single figure is a representation of a typical electrolysis cell. In that figure, the numeral 1 identifies a container which is divided into three compartments 6, 7 and 8 by an anionic permselective diaphragm 2, which is adjacent to the anode 4, and by a cationic permselective diaphragm 3, which is adjacent to the cathode 5. The ionic-permselective diaphragms are described in greater detail below. Compartment 6 is the anode compartment because it contains anode 4 while compartment 7 is the cathode compartment because it contains the cathode 5. The numeral 8 identifies the third and intermediate compartment into which is placed the solution containing the ions of the amphoteri'c and non-amphoteric metals to be separated. When the cell is in operation, the electrodes 4 and are connected to a source of electric power not shown.

In the operation of this process an aqueous solution of an acid, preferably sulfuric acid, is placed in compartment 6, and an aqueous solution of a strong base such as an. hydroxide of an. alkali metal, preferably sodium hydroxide, is placed in compartment 7, and an aqueous solution of the ionizable compoundsordinarily salts of the amphoteric and non-amphoteric metals is placed in compartment 8. As a direct current is passed through the cell the ions in all of the compartments tend to migrate to the electrode of opposite charge. Thus, ions of the non-amphoteric metal migrate from the intermediate compartment 8 through the cationic permselective membrane 3 into the cathode compartment where hydrogen is liberated and hydroxyl ions are formed. Here in the case of the group I metals a soluble and ionized hydroxide is formed while in the case of other metals, e. g., group II metals, an insoluble'hydroxide is precipitated, all in accordance with established laws of electrolysis. Migration of the anions in the center compartment into the anode compartment also takes place and the anions are discharged in accordance with the laws of electrolysis. Thus, when they are sulfate ions, oxygen is liberated at the anode and hydrogen ions are formed in solution with the net result that the concentration of the ions of sulfuric acid increases progressively in the anode compartment. If, however, the anions of the salts in the intermediate compartment are chloride ions, they are discharged at the anode and gaseous chlorine is liberated. The hydroxyl ions in the cathode compartment tend to migrate to the anode but they are constrained by the cationic diaphragm 3. Likewise, hydrogen ions in the anode compartment tend to migrate to the cathode but are constrained by the anionic permselective diaphragm 2. Surprisingly, the ions of the amphoteric metal in the intermediate compartment do not migrate in any substantial amount through the cationic permselective diaphragm 3, as they would normally be expected to do. It is believed that the positive ions of the amphoteric metal are converted to negative ions, for example, zincate ions, on coming in contact with hydroxyl ions which have diffused to a slight extent into membrane 3. The negative ions of the amphoteric metal then migrate in the opposite direction toward the anode but do not enter the anode compartment 6 because apparently they are reconverted to cations on coming in contact with hydrogen ions which have diffused to a limited extent into the anionic permselective diaphragm 2. Such is the theory. But the facts here are independent of the theory and the net result in any case is that the ions of the amphoteric metal remain in the intermediate compartment in substantially quantitative amounts as the electrolysis proceeds.

Thus, the process provides a means of separating ions of certain amphoteric metals from ions of non-amphoteric metals.

The value of this invention can be appreciated from a consideration of one particular instance where it is most desirable to effect such a separation. For example, in the production of rayon by the viscose process large quantities of sodium hydroxide are required to convert the cellulose into soda cellulose or alkali cellulose. Later in the coagulating baths, which contain both sulfuric acid and zinc sulfate, the sodium of the soda cellulose is converted to sodium sulfate. This gives rise, therefore, to a solution of sodium sulfate and zinc sulfate. Attempts have been made heretofore to convert the zinccontaminated sodium sulfate to sodium hydroxide by electrolysis with the idea of using the sodium hydroxide in producing more soda cellulose. But, for the most part, the results of such attempts have not been satisfactory because the sodium hydroxide so produced was also contaminated with zinc and this contaminant had an adverse effect on the physical properties of the soda cellulose produced from the contaminated sodium hydroxide. Thus, the problem became one of separating zinc salts from sodium salts.

By the process of this invention, however, sodium hydroxide is produced which contains so little zinc--if any -that it can be used with complete satisfaction in the viscose process.

The cell which is employed in the process of this invention can be varied as to size, closures, kinds of electrodes, construction materials, controls, siZe of the individual compartments, means for supplying aqueous solutions to the various compartments, means for removing the aqueous solutions from the compartments, embellishments, et cetera. What is essential, however, is that the cell have three compartments, one containing an anode, another containing a cathode and a third intermediate compartment which is separated from the anode compartment by means of an anionic permselective mem brane and is separated from the cathode compartment by means of a cationic permselective membrane.

The permselective membranes which divide the electrolysis cell into the three compartments are all important to the success of this process. They function by allowing only one kind of ions, either anions or cations, to pass through them while at the same time preventing-or at least restrainingthe passage of the other kind of ions through them from one compartment to another. The anionic membrane contains an anion-exchange resin and the cationic membrane contains a cation-exchange resin.

The composition of the ionic permselective membranes can vary within reasonable limits but it is essential to this invention that the membranes contain enough resin so as to have suitably high conductance when employed in an electrolysis cell. The permselective films which have proven to be most suitable for use in this process are those made by incorporating particles of ion-exchange resin in a film-forming matrix such as polyethylene, polyvinyl chloride, natural or synthetic rubber. Such films are the subject of my applications, Serial Nos. 202,577 and 205,413 (now Patents Nos. 2,681,320 and 2,681,319 respectively), to which reference is made, and they contain from 25% to 75% of the ion-exchange resins on a weight basis. They are physically strong and flexible so that they can be mounted easily in a cell. Other permselective films are known such as those based on cellophane or collodion; but those do not contain ion-exchange resins and are not recommended for use in the instant invention because they are not sufficiently chemicalresistant.

What are required here are films or layers containing in one case a cation-exchange resin and in the other case an anion-exchange resin. Cation-exchange resins are well known and are widely used in the removal of ions from fluids. Suitable cation-exchange resins are described in U. 5. Patents Nos. 2,184,943; 2,195,196; 2,204,539; 2,228,159; 2,228,160; 2,230,641; 2,259,455; 2,285,750; 2,319,359; 2,366,007; 2,340,110; and 2,340,111. Some of the resins can be cast or otherwise produced in the form of free sheets or membranes. Or the resins can be made on a porous support such as a piece of cloth or plastic screening. Of all of the cationic permselective membranes which have been employed, the ones which are preferred are those containing a sulfonated copolymer of styrene and divinylbenzene. Suitable anion-exchange resins are described in U. S. Patents Nos. 2,106,486; 2,151,883; 2,223,930; 2,251,234; 2,259,169; 2,285,750; 2,341,907; 2,354,671; 2,354,672; 2,356,151; 2,366,008; 2,388,235; 2,402,384; 2,591,573; and 2,591,574.

The words membrane, film, sheet, layer, pellicle and diaphragm are used synonymously herein to describe the barriers or partitions between the compartments in the electrolysis cell. The barriers are usually thin-of the order of thickness of 20 to 100 milsalthough thicker membranes have been used successfully.

The electric current is direct. The current density can, of course, be varied and just what current density is maintained depends upon the construction of the cell and on other prevailing conditions of operation. Current densities from 50 to 150 amperes per square foot of area of either permselective membrane have, however, been employed very successfully.

The following example is given as a means of illustrating the process of this invention.

Example A cell, equipped with platinum electrodes and similar to that in the drawing, was used. The anolyte was a 10% aqueous solution of sulfuric acid while the catholyte was a 4% aqueous solution of sodium hydroxide. Into the center compartment was charged an aqueous solution containing 26.8% sodium sulfate and 0.66% Zinc sulfate. The anionic permselective membrane was composed of of an anion-exchange resin dispersed uniformly and intimately in a matrix of polyethylene and the cationic permselective membrane contained of a cation-exchange resin also dispersed in polyethylene. The anion-exchange resin itself was of the strongly basic quaternary ammonium type made by chloromethylating a copolymer of 96% styrene and 4% divinylbenzene and reacting the chloromethylated product with trimethylamine according to the process of U. S. Patent No. 2,591,573. The cation-exchange resin itself was a sulfonated cross-linked copolymer of styrene and divinylbenzene made by the process of U. S. Patent No. 2,366,007.

The anionic membrane was mounted adjacent to the anode while the cationic membrane was mounted adjacent to the cathode. A direct current was passed through the cell for five hours and fifty minutes at a median current density of amperes per square foot. At the end of this time, the contents of the three compartments were removed and analyzed. It was found that 89% of the sodium sulfate, originally in the center compartment, had been converted into sodium hydroxide in the cathode compartment, and into sulfuric acid in the anode compartment. No zinc whatever was found in either the anode compartment or the cathode compartment. The zinc had been quantitatively retained in the center compartment. The power consumed was very close to 5 kilowatt hours per pound of sodium hydroxide produced.

Essentially the same results were obtained when mixtures containing higher ratios of Zinc sulfate to sodium sulfate were electrolyzed in the same way. Thus, when mixtures, containing in one case 26.8% NazSOr and 6.6% zinc sulfate and in another case 7% Na2SO4 and 7.8% zinc sulfate, were electrolyzed, all of the zinc was retained in the center compartment.

While the process of this invention has been exemplified by the separation of zinc and sodium ions, with the pro duction of zinc-free sodium hydroxide and sulfuric acid from zinc contaminated sodium sulfate, it is true that this process has much wider application. Thus, it has been found to provide a means for the eflicient production of the hydroxides of the alkali metals of group I of the Periodic Table and of the non-amphoteric metals of group II from aqueous solutions of salts of these metals and salts of aluminum, as well as zinc. Also ammonium ions are separated in the same way from ions of. the amphoteric metals. And all of the hydroxides thus produced can, in turn, be readily converted to pure salts.

I claim:

1. An electrolytic process for the separation of ions of an alkali metal from an aqueous solution containing ions of said alkali metal together with ions of an amphoteric metal from the class consisting of zinc and aluminum which comprises passing a direct electric current through an electrolysis cell which has (a) an anode compartment in which there is an aqueous solution containing hydrogen ions, (b) a cathode compartment in which there is an aqueous solution of the hydroxide of said alkali metal and (c) an intermediate compartment in which there is an aqueous solution containing ions of both said alkali metal and said amphoteric metal, said intermediate compartment being separated from said anode compartment by an anionic permselective diaphragm which contains an anion-exchange resin and being separated from said cathode compartment by a cationic permselective diaphragm whifih Contains a cation-exchange resin.

2. An electrolytic process for the separation of ions of sodium from an aqueous solution containing ions of sodium and ions of zinc which comprises passing a direct electric current through an electrolysis cell which has (a) an anode compartment in which there is an aqueous solution containing hydrogen ions, (b) a cathode compartment in which there is an aqueous solution of sodium hydroxide and (c) an intermediate compartment in which there is an aqueous solution containing ions of sodium and ions of zinc, said intermediate compartment being separated from said anode compartment by an anionic permselective diaphragm which contains an anion-exchange resin and being separated from said cathode compartment by a cationic permselective diaphragm which contains a cation-exchange resin.

3. A process for preparing substantially zinc-free sodium hydroxide and sulfuric acid from a mixture of sodium sulfate and zinc sulfate which comprises passing a direct electric current through an electrolysis cell which has an anode compartment containing aqueous sulfuric acid, a cathode compartment containing aqueous sodium hydroxide and an intermediate compartment containing an aqueous solution of sodium sulfate and zinc sulfate, said intermediate compartment being separated from said anode compartment by an anionic permselective diaphragm which contains an anion-exchange resin and being separated from said cathode compartment by a cationic permselective diaphragm which contains a cationexchange resin.

4. The process of claim 3 in which the anionic permselective diaphragm contains a strongly basic quaternary ammonium anion-exchange resin made by first chloromethylating and then aminating with a tertiary amine an insoluble cross-linked copolymer of styrene and divinylbenzene and in which the cationic permselective diaphragm contains a cation-exchange resin which is a sulfonated copolymer of styrene and divinylbenzene.

References Cited in the file of this patent UNITED STATES PATENTS 2,636,851 Iuda et a1 Apr. 28, 1953 2,636,852 Juda et a1. Apr. 28, 1953 OTHER REFERENCES Chemical Abstracts, vol. 37 (1943), page 2274, abstract of article entitled Regeneration of the rayon spinning bath by electrolysis. (Copy in Sci. Lib.) 

1. AN ELECTROLYTIC PROCESS FOR THE SEPARATION OF IONS OF AN ALKALI MATAL FROM AN AQUEOUS SOLUTION CONTAINING IONS OF SAID ALKALI METAL TOGETHER WITHIONS OF AN AMPHOTERIC METAL FROM THE CLASS CONSISTING OF ZINC AND ALUMINUM WHICH COMPRISES PASSING A DIRECT ELECTRIC CURRENT THROUGH AN ELECTROLYSIS CELL WHICH HAS (A) AN ANODE COMPARTMENT IN WHICH THERE IS AN AQUEOUS SOLUTION CONTAINING HYDROGEN IONS, (B) A CATHODE COMPARTMENT IN WHICH THERE IS AN AQUEOUS SOLUTION OF THE HYDROXIDE OF SAID ALKALI METAL AND (C) AN INTERMEDIATE COMPARTMENT IN WHICH THERE IS AN AQUEOUS SOLUTION CONTAINING IONS OF BOTH SAID ALKALI METAL AND SAID AMPHOTERIC METAL, SAID INTERMEDIATE COMPARTMENT BEING SEPERATED FROM SAID ANODE COMPARTMENT BY AN ANIONIC PERMSELECTIVE DIAPHRAGM WHICH CONTAINS ANN ANION-EXCHANGE RESIN AND BEING SEPERATED FROM SAID CATHODE COMPARTMENT BY A CATIONIC PERMSELECTIVE DIAPHRAGM WHICH CONTAINS A CATION-EXCHANGE RESIN. 